Internal combustion engines generally produce engine output torque by performing combustion in the engine cylinders. Specifically, each cylinder of the engine inducts air and fuel and combusts the air-fuel mixture, thereby increasing pressure in the cylinder to generate torque to rotate the engine crankshaft via the pistons. One method to improve engine fuel economy is to deactivate a selected group of cylinders to thereby raise manifold pressure and reduce pumping work of the remaining cylinders carrying out combustion. The cylinder deactivation can be accomplished by mechanically deactivating the intake and exhaust valves of the selected cylinders.
The inventors herein have recognized a problem with such an approach. Specifically, during the cylinder deactivation mode, the imbalance between the torque produced in cylinders carrying out combustion, and the torque of the deactivated cylinders can cause increased engine vibration and harshness. Such vibration and harshness can be experienced by the vehicle operator and thereby reduce vehicle drive feel.
One approach to reduce the engine torque variation caused by deactivated cylinders is described in U.S. Pat. No. 6,332,446. In this method, where particular cylinders are deactivated under cylinder deactivation control, the exhaust valve of each deactivated or inactive cylinder is opened for a certain period of time which starts ahead of the bottom dead center of piston movement. The timing of opening the exhaust valves is determined so that the pressure within the inactive cylinder is equal to or lower than the atmospheric pressure when the exhaust valve is open. The timing of closing the exhaust valve is determined so that the peak value of the pressure within the inactive cylinder becomes almost equal to the peak value of pressure within active cylinders.
The inventors of the present invention have recognized still further disadvantages with the approach of U.S. Pat. No. 6,332,446. Specifically, the method utilized for cylinder deactivation in this approach can degrade fuel economy since additional power is utilized to open and close the exhaust valve in the deactivated cylinders. In other words, energy is utilized to open and close the exhaust valve in deactivated cylinders, yet the cylinders do not produce any significant net torque to assist engine rotation. As such, even when the torque variation caused by cylinder deactivation may be unnoticed by the driver, energy is spent opening and closing the exhaust valve of deactivated cylinders thereby degrading overall engine efficiency. In addition to the electrical losses associated with opening and closing the valves, further fuel economy degradation occurs because the heat and mass losses from the trapped gas in the cylinders increase due to the higher pressures and temperatures in the non-firing cylinders. Thus, further fuel is expended in the firing cylinders to overcome the parasitic gas cycle losses in the non-firing cylinders.
Finally, in the case where valves are opened at lower than atmospheric pressure, such operation can degrade fuel economy as it would increase the gas cycle losses in the inactive cylinders due to unrestrained expansion of the exhaust gas back into the cylinders.
Still another disadvantage of the approach described in U.S. Pat. No. 6,332,446 relates to increased oil consumption. In other words, in the case where the method of U.S. Pat. No. 6,332,446 requires pressure in the inactive cylinder to be lower than the atmospheric pressure during some conditions, the cylinder pressure is therefore also lower than the engine crank case pressure in some conditions. As such, the positive pressure in the crank case causes oil to transfer to the cylinder, which oil is in turn later burned during combustion causing increased engine emissions.